Restore special case for Vector.++ to detect prepends

scala · Jun 5, 2020 · 5d2b94c · 5d2b94c
1 parent e4e3596
commit 5d2b94c
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diff --git a/src/library/scala/collection/immutable/Vector.scala b/src/library/scala/collection/immutable/Vector.scala
@@ -198,6 +198,7 @@ sealed abstract class Vector[+A] private[immutable] (private[immutable] final va
     if(k == 0) this
     else appendedAll0(suffix, k)
   }
+  private final val Log2ConcatFaster = 5
 
   protected[this] def appendedAll0[B >: A](suffix: collection.IterableOnce[B], k: Int): Vector[B] = {
     val tinyAppendLimit = 4 + vectorSliceCount
@@ -208,6 +209,11 @@ sealed abstract class Vector[+A] private[immutable] (private[immutable] final va
         case _ => suffix.iterator.foreach(x => v = v.appended(x))
       }
       v
+    } else if (this.size < (k >>> Log2ConcatFaster) && suffix.isInstanceOf[Vector[B]]) {
+      var v = suffix.asInstanceOf[Vector[B]]
+      val ri = this.reverseIterator
+      while (ri.hasNext) v = v.prepended(ri.next())
+      v
     } else new VectorBuilder[B].initFrom(this).addAll(suffix).result()
   }
 

diff --git a/test/junit/scala/collection/immutable/VectorTest.scala b/test/junit/scala/collection/immutable/VectorTest.scala
@@ -412,6 +412,12 @@ class VectorTest {
       assertEquals(0 until (size + appendSize), v3)
     }
   }
+
+  // scala/bug#12027
+  // This took forever in 2.13.2
+  @Test def testAppendWithTinyLhs(): Unit = {
+    (1 to 1000000).foldLeft(Vector.empty[Int]) { (v, i) => Vector(i) ++ v }
+  }
 }
 
 object VectorUtils {